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    When \(a \ne 0\), there are two solutions to \(ax^2 + bx + c = 0\) and they are 
	$$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$
	$$若积分曲面\Sigma能表示为z=z(x,y),并且在xOy平面上的投影域为D,则$$
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